Many optical instruments are known which interrogate the eye with beams of light for determining the optical properties of the eye. Almost all of such instruments require accurate towards and away positioning of the eye from the instrument itself. That is to say the eye must be located at a precise distance from the objective of the interrogating instrument for the interrogating beam to make accurate measurement. Placement of the eye either too close to or too far away from the instrument renders an unreliable optical reading.
Examples of such instruments are disclosed in Humphrey U.S. Pat. No. 4,407,572 entitled Keratometer issued June 12. 1980; Humphrey U.S. Pat. No. 4,560,259 entitled Objective Refractor for the Eye issued Dec. 24, 1985: and Humphrey U.S. Pat. No. 4,732,466 entitled Fundus Camera issued Apr. 4, 1988.
Many of these instruments include possible schemes for determining the towards and away range of the eye along the interrogating axis. However, such schemes lack the overall reliability and simplicity needed for rapid ranging of the eye. Since the instruments attempt to determine the distance to the eye from interrogation parallel to the interrogating light, accuracy has been hard to obtain.